This invention pertains in general to fuel elements for nuclear reactors, and more particularly, to internally pressurized, clad fuel elements which generate fission gases during burn-up.
Many designs of presently manufactured commercial nuclear reactors have their fuel inventory of fissile and fertile material contained within a plurality of elongated metallic clad fuel elements or fuel rods. The fuel rods comprise a tubular cladding member hermetically closed by a pair of end plugs. The fuel generally consists of juxtaposed ceramic pellets, of for example uranium dioxide, contained within the metallic cladding. The exterior of the cladding is exposed to high temperture, high pressure environment. For example, in a pressurized water reactor, a fuel element will be exposed to pressures in excess of 2,000 psi and temperatures above 500.degree.F.
A recent innovation in fuel element manufacture has been to internally pressurize the fuel rods before insertion into the core of a nuclear reactor. This increase in the designed internal pressure of the fuel rods during manufacture offsets the external pressure on the cladding walls in the reactor core during reactor operation and thereby reduces the stresses on the fuel cladding. The reduction in cladding stresses facilitates the manufacture of fuel elements with thinner walls which during reactor operation aids the escape of neutrons to other fuel rods, thus increasing the neutron economy and thereby reducing the cost of reactor operation.
During burn-up of the fissile fuel pellets in the operating reactor, fission gases are released which increase the internal pressure within the fuel rods above the design pressure provided during fuel rod manufacture. Although internal pressurization during fuel rod manufacture greatly reduces the differential pressure across the cladding during reactor operation and thereby increases the reliability of such fuel rods, a problem is created after substantial burnup of the fuel due to the amount of fission gases which accumulate within such fuel rods. This build-up effects the internal and external pressure balance achieved by internal pressurization of the fuel rods during the manufacturing process, thus rendering the fuel rod cladding susceptible to rupture. Thus, in order to manufacture reliable fuel elements and particularly such fuel elements as are to be pressurized during manufacture (hereinafter referred to as pressurized fuel elements), some means must be provided for accommodating the fission gases released during burn-up. An example of such a pressurized fuel element with fission gas accommodating means may be found in U.S. Pat. No. 3,664,174, issued Feb. 22, 1972 by H. M. Ferrari and assigned to the Westinghouse Electric Corporation.
Another solution to the above problem of fission gas accumulation may be found in U.S. Pat. No. 3,647,622, issued Mar. 7, 1972, by H. N. Andrews et al, and assigned to the Westinghouse Electric Corporation. According to this latter application, one or more bellows like members are provided within the interior of the fuel element, each having an internally mounted pin. As fission gases accumulate the various bellows are punctured providing additional void space for fission gas accumulation. However, in both of the above cited applications, plots of the internal pressure versus time for such fuel rods show a saw tooth pattern with deeply decreasing pressure gradients at the predetermined points of internal bellows failure or puncture. A cyclic differential pressure pattern is thereby produced on the cladding. It is possible that the sudden increase in differential pressure, as each bellows intentionally fails, might lead to reduced reliability of the fuel rods, especially where a cladding material having a low modulus of elasticity is used. Unfortunately, materials such as zircaloy which are desirable for use as cladding in nuclear fuel elements because of their relatively low capture cross-section with respect to thermal neutrons, generally have a low modulus of elasticity.
Another solution of the prior art may be found in application Ser. No. 802,544, filed Feb. 26, 1969, by H. M. Ferrari and M. B. L. Hepps and assigned to the Westinghouse Electric Corporation. Here it is suggested that a bellows-like member be internally supported within the plenum of the nuclear fuel element and communicably coupled with the environment of the fuel element so as to maintain an internal pressure substantially equal to the external environmental pressure during burn-up and load follow. The bellows-like member is preferably constructed of a material with a relatively high modulus of elasticity so that it may readily expand or contract to compensate for the amount of fission gas released by the fuel during burn-up. While the latter arrangement is an improvement over the prior art, it has the disadvantage that, if the bellows should fail, coolant will be exposed to the interior of the fuel element, and the fission gases will be released into the coolant.
An additional solution of the prior art which provides for substantially linear compensation of the internal pressure build-up within the fuel element may be found in application Ser. No. 084,302, filed Oct. 27, 1970, by Raymond J. Bratton et al, and assigned to the Westinghouse Electric Corporation. This application provides an internally pressurized hermetically clad fuel element for a nuclear reactor having a normally sealed chamber within the fuel element plenum. The chamber is maintained at a lower pressure than the internal pressure of the fuel element and at least a portion of one of the chamber walls is formed from a permeable membrane material which allows the plenum gases to diffuse into the chamber at a rate comparable to the rate of fission gas release from the fuel during burn-up. The complexity in the design of the membrane and chamber to adequately compensate for the internal pressure build-up detracts from the value of the invention in maintaining fuel element pressure control.